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Abstract

In a conventional digital phase conjugation system, only the phase of an input light is time-reversed. This deteriorates phase conjugation fidelity and restricts application fields to specific cases only when the input light has uniformly-distributed scattered wavefront. To overcome these difficulties, we present a digital phase conjugate mirror based on parallel alignment of two phase-only spatial light modulators (SLMs), in which both amplitude and phase of the input light can be time-reversed. Experimental result showed that, in the phase conjugation through a holographic diffuser with diffusion angle of 0.5 degree, background noises decrease to 65% by our digital phase conjugation mirror.

Fig. 2 Experimental setup for dual-phase modulation method. Reference light is interfered with generated light for measurement of complex amplitude with holographic diversity interferometer (HDI). The HDI consists of two CCDs, PBS, and QWP. QWP makes phase difference of π/2 between transmitted light and reflected lights after PBS2, which is necessary for phase measurement with the HDI. Phase-only SLM provided by Hamamatsu can modulate the phase in the range [0, 2π] without any change of intensity. PSLM1 and PSLM2 offer phase modulation of 2π when gray levels are 85 and 157.

Fig. 5 Independent modulation of amplitude image and phase image with the same resolution as that of PSLM. Each image consists of 256 × 256 SLM pixels. (a) Desired amplitude image. This is represented by 256 gray levels and includes all gray levels. (b) Desired phase image. This is represented by 256 gray levels and includes all gray levels. (c) Measured amplitude image. SNR was 6.25 dB. (d) Measured phase distribution when only the amplitude image is modulated. (e) Measured phase image. SNR was 5.85 dB. This low value is mainly due to phase wrapping. (f) Measured amplitude distribution when only the phase image is modulated.

Fig. 8 Experimental setup for digital phase conjugate mirror. A OHP sheet is overhead projector sheet used for the correct mapping between SLMs and CCDs. The amplitude object is USAF resolution target (Edmund) and it is arranged at the focal plane of L1. A phase object is a plane-convex lens with f = 500mm or a holographic diffuser with diffusion angle of 0.5 degree (Edmund) and it is arranged at a distance of 30 mm from an amplitude object.